CsLiLn HALIDE SCINTILLATOR

ABSTRACT

Li-containing scintillator compositions, as well as related structures and methods are described. Radiation detection systems and methods are described which include a Cs 2 LiLn Halide scintillator composition.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/603,989 (now U.S. Pat. No. 9,459,357), filed Jan. 23, 2015,which is a continuation of U.S. patent application Ser. No. 13/891,771(now U.S. Pat. No. 8,969,824), filed May 10, 2013, which is acontinuation of U.S. patent application Ser. No. 12/731,003 (now U.S.Pat. No. 8,440,980), filed Mar. 24, 2010 which is a continuation-in-partapplication of U.S. patent application Ser. No. 12/624,337, filed onNov. 23, 2009, which claims the benefit of priority under 35 U.S.C.§119(e) of U.S. Provisional Application No. 61/230,970, filed Aug. 3,2009, which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to scintillator compositions and relateddevices and methods. More specifically, the present invention relates toenriched Li-containing scintillator compositions suitable for use, forexample, in radiation detection, including gamma-ray spectroscopy, andX-ray and neutron detection.

Scintillation spectrometers are widely used in detection andspectroscopy of energetic photons (e.g., X-rays, gamma-rays, etc.). Suchdetectors are commonly used, for example, in nuclear and particlephysics research, medical imaging, diffraction, non destructive testing,nuclear treaty verification and safeguards, nuclear non-proliferationmonitoring, and geological exploration.

Important requirements for the scintillation crystals used in theseapplications include high light output, transparency to the light itproduces, high stopping efficiency, fast response, good proportionality,low cost, and availability in large volume. These requirements on thewhole cannot be met by many of the commercially available scintillatorcompositions. While general classes of chemical compositions may beidentified as potentially having some attractive scintillationcharacteristic(s), specific compositions/formulations having bothscintillation characteristics and physical properties necessary foractual use in scintillation spectrometers and various practicalapplications have proven difficult to predict. Specific scintillationproperties are not necessarily predictable from chemical compositionalone, and preparing effective scintillator compositions from evencandidate materials often proves difficult For example, while thecompositions of sodium chloride and sodium iodide had been known formany years, the invention by Hofstadter of a high light-yield andconversion efficiency scintillator from sodium iodide doped withthallium launched the era of modern radiation spectrometry. More thanhalf a century later, thallium doped sodium iodide, in fact, stillremains one of the most widely used scintillator materials. Since theinvention of NaI(Tl) scintillators in the 1940's, for half a centuryradiation detection applications have depended to a significant extenton this material. The fields of nuclear medicine, radiation monitoring,and spectroscopy have grown up supported by NaI(Tl). Although far fromideal, NaI(Tl) was relatively easy to produce for a reasonable cost andin large volume. With the advent of X-ray CT in the 1970's, a majorcommercial field emerged as did a need for different scintillatorcompositions, as NaI(Tl) was not able to meet the requirements of CTimaging. Later, the commercialization of positron emission tomography(PET) imaging provided the impetus for the development of yet anotherclass of detector materials with properties suitable for PET.

As the methodology of scintillator development evolved, new materialshave been added, and yet, specific applications are still hampered bythe lack of scintillators suitable for particular applications. Today,the development of new scintillator compositions continues to be as muchan art as a science, since the composition of a given material does notnecessarily determine its properties as a scintillator, and becausescintillation properties are strongly influenced by the history (e.g.,fabrication process) of the material as it is formed.

Thus, there is continued interest in the search for new and usefulscintillator compositions and formulations, as well as correspondingdetection systems, with both the performance and the physicalcharacteristics needed for use in various applications.

BRIEF SUMMARY OF THE INVENTION

The present invention provides Li-containing scintillator compositions,as well as related structures and methods. Compositions include Cs2LiLnHalide (Z₆) scintillator compositions.

In one aspect, structures and methods of the present invention include ascintillator comprising a Cs₂LiLn Halide composition. Ln is selectedfrom one or more of Y, La, Ce, Gd, Lu and Sc, and the halide comprisesat least Cl. The scintillator is capable of neutron detection at anefficiency of greater than 30%. In some of these embodiments. In someembodiments, the lithium content of the composition is enriched toinclude a Li-6 content above that which is found in naturally occurringlithium sources. In some embodiments, the scintillator compositionsdisclosed herein can include a dopant or a mixture of dopants.

In one aspect, a detection system is provided. The system comprises ascintillator comprising a Cs₂LiLn Halide composition. Ln is selectedfrom one or more of Y, La, Ce, Gd, Lu and Sc. The Halide comprises atleast Cl, wherein the scintillator is capable of neutron detection at anefficiency of greater than 30%. The system further comprises a detectorassembly coupled to the scintillator to detect a light pulseluminescence from the scintillator as a measure of a neutronscintillation event.

In one aspect, a method of radiation detection is provided. The methodcomprises providing a detection system comprising a scintillatorcomprising Cs₂LiLn Halide composition. Ln is selected from one or moreof Y, La, Ce, Gd, Lu and Sc, wherein the Halide comprises at least Cl.The detection system further comprises a detection assembly coupled tothe scintillator to detect a light pulse luminescence from thescintillator as a measure of a scintillation event. The method furthercomprises positioning the system such that a radiation source is withina field of view of the system so as to detect emissions from the sourceand measuring a scintillation event luminescence signal from thescintillator with the detection assembly. The method further comprisesprocessing the measured luminescence signal using pulse shapediscrimination analysis over a time of greater than 50 ns todifferentiate between gamma emissions and neutron emissions from thesource.

In some aspects, a method of radiation detection is provided. The methodcomprises providing a detection system comprising a scintillatorcomprising Cs₂LiLn Halide composition. Ln is selected from one or moreof Y, La, Ce, Gd, Lu and Sc, wherein the Halide comprises at least Cl.The detection system further comprises a detection assembly coupled tothe scintillator to detect a light pulse luminescence from thescintillator as a measure of a scintillation event. The method furthercomprises positioning the system such that a radiation source is withina field of view of the system so as to detect emissions from the sourceand measuring a scintillation event luminescence signal from thescintillator with the detection assembly. The method further comprisesprocessing the measured luminescence signal comprising comparing themeasured luminescence signal from a first window of time to the measuredluminescence signal from a second window of time.

Excellent scintillation properties, including high light output, goodproportionality, response, and good energy resolution have been measuredfor certain compositions of the present invention. Scintillatorcompositions of the present invention have demonstrated emissioncharacteristics indicating suitability for use in various applications.For example, scintillation properties of the compositions can includepeak emission wavelengths from about 380 nm, which is well matched toPMTs as well as silicon diodes used in nuclear instrumentation and apeak wavelength for gamma-ray spectroscopy.

The present invention, in some aspects, advantageously provideshigh-efficiency neutron detection compositions and structures. Thus,compositions of the present invention may be used in a variety ofradiation detection structures and applications.

Scintillator compositions demonstrated suitability for gamma-rayspectroscopy and neutron emission detection, including differentialgamma-ray/neutron detection. Surprisingly good energy resolution of thecompositions make the scintillators of the present inventionparticularly attractive for combined or simultaneous gamma and neutrondetection. Additionally, timing characteristics such as rise time anddecay time for gamma-ray and neutron may be utilized in differentialdetection of gamma-ray scintillation events and neutron events. In oneembodiment, detection includes measuring and/or processing ascintillation luminescence signal including comparing different timingwindows so as to identify a scintillation event as a gamma event orneutron event.

In another aspect, the invention further includes systems and devicesmaking use of the scintillator compositions of the present invention. Asystem or device can include, for example, a radiation detection devicehaving a scintillation composition as described herein, and a detectorassembly optically coupled to the scintillator composition. A detectorassembly can include, for example, a photomultiplier tube, a photodiode, or a PIN detector. The detector assembly may further include adata analysis, or computer, system for processing and analyzing detectedsignals. Exemplary devices or assemblies can include an X-ray and/orneutron detector assembly, as well as imaging systems. For example, thedevice can include electronics configured for performing pulse shape andpulse height analysis to differentiate gamma ray from neutron emissions.Scintillator compositions of the present invention can further find usein a variety of detector or imaging system configurations commonly usingscintillator compositions, and methods of the present invention caninclude radiation detection and/or imaging applications using theaforementioned devices/systems.

In yet another aspect, the invention includes a method of performingradiation detection. Such a method can include, for example, providing adetection system or device having a scintillator composition of thepresent invention, and the system or a target/radiation source such thatthe source is within a field of view of the scintillator for detectingemissions from the target or source. Emissions can include, for example,gamma-ray, X-ray, or neutron emissions. A target can include variouspotential sources of detectable emissions including neutron emitters andgamma-ray sources (e.g., uranium and the like), X-ray sources, etc.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings. The drawingsrepresent embodiments of the present invention by way of illustration.The invention is capable of modification in various respects withoutdeparting from the invention. Accordingly, the drawings/figures anddescription of these embodiments are illustrative in nature, and notrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows neutron absorption efficiency as a function of scintillatorthickness for a non-enriched Cs₂LiYCl₆ and a Li-6 enriched Cs₂LiYCl₆scintillator composition.

FIG. 2 shows time profiles for gamma-ray and neutron detection for aCs₂LiYCl₆ scintillator composition, according to an embodiment of thepresent invention.

FIG. 3 shows time profiles for gamma-ray and neutron detection for ascintillator composition, according to an embodiment of the presentinvention, with a first timing window and a second timing window fordifferentiation of gamma-ray and neutron scintillation event signal.

FIG. 4 illustrates comparison of different portions of scintillationevent signal, as in FIG. 3, for differentiation of gamma-ray and neutronevents, according to one embodiment of the present invention.

FIG. 5 shows an optical emission spectrum for a Ce doped, Cs₂LiYCl₆composition upon gamma-ray irradiation, according to an embodiment ofthe present invention.

FIG. 6 illustrates proportionality for a Ce doped, Cs₂LiYCl₆composition, according to an embodiment of the present invention. Thefigure shows light output of the composition measured under excitationfrom isotopes such as ²⁴¹Am, ⁵⁷Co, ²²Na, and ¹³⁷Cs.

FIG. 7 illustrates an energy spectrum for a Ce doped, Li-6 enrichedCs₂LiYCl₆ composition, according to an embodiment of the invention. Thefigure shows excellent energy resolution measured for the composition.

FIG. 8A is a conceptual diagram of a radiation detection system of thepresent invention.

FIG. 8B is a diagram of a scintillator composition disposed on asubstrate, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention will be better understood with resort to the followingdefinitions:

A. Rise time, in reference to a scintillation crystal material, shallmean the speed with which its light output grows once a gamma-ray hasbeen stopped in the crystal. The contribution of this characteristic ofa scintillator contributes to a timing resolution.

B. A Fast timing scintillator (or fast scintillator) typically requiresa timing resolution of about 500 ps or less. For certain PETapplications (e.g., time-of-flight (TOF)), the fast scintillator shouldbe capable of localizing an annihilation event as originating fromwithin about a 30 cm distance, i.e., from within a human being scanned.

C. Timing accuracy or resolution, usually defined by the full width halfmaximum (FWHM) of the time of arrival differences from a point source ofannihilation gamma-rays. Because of a number of factors, there is aspread of measured values of times of arrival, even when they are allequal. Usually they distribute along a bell-shaped or Gaussian curve.The FWHM is the width of the curve at a height that is half of the valueof the curve at its peak.

D. Light Output shall mean the number of light photons produced per unitenergy deposited by the detected gamma-ray, typically the number oflight photons/MeV. For neutrons, the light output is typically measuredin photons/neutron.

E. Stopping power and attenuation refer to the penetration range of theincoming X-ray or gamma-ray in the scintillation crystal material.Attenuation is the gradual loss of intensity of a flux through a medium.The attenuation length, in the context of a scintillator, is the lengthof scintillator material needed to reduce the incoming beam flux to1/e⁻. For neutrons, the attenuation length and the useful attenuationlength may differ. For instance, for Cs₂LiYZ₆ scintillator compositions,neutrons are stopped by all of the elements in the composition, withthose stopped by Li-6 provide a useful signal.

F. Proportionality of response (or linearity). For some applications(such as CT scanning) it is desirable that the light output besubstantially proportional to the deposited energy. For applicationssuch as spectroscopy, non-proportionality of response is an importantparameter. In a typical scintillator, the number of light photonsproduced per MeV of incoming gamma-ray energy is not constant. Rather,it varies with the energy deposited by the stopped gamma-ray. This hastwo deleterious effects. The first is that the energy scale is notlinear, but it is possible to calibrate for the effect. The second isthat it degrades energy resolution. To see how this occurs, consider ascintillator that produces 300 photons at 150 keV, 160 photons at 100keV and 60 photons at 50 keV. From statistics alone, the energyresolution at 150 keV should be the variability in 300 photons, which is5.8%, or 8.7 keV. If every detected event deposited 150 keV in one stepthis would be the case. On the other hand, if, as it occurs, an eventdeposited 100 keV in a first interaction and then another 50 keV in asecond interaction, the number of photons produced would not be 300 onthe average, but 160+60=220 photons, for a difference of 80 photons or27%. In multiple detections, the peak would broaden well beyond thetheoretical 8.7 keV. The smaller the non-proportionality the smallerthis broadening and the closer the actual energy resolution approachesthe theoretical limit.

The present invention includes compositions and related radiationdetection systems incorporating a Cs₂LiLn Halide composition. Thecompositions may be represented by the formula Cs₂LiLnZ₆, where Z is ahalide. Suitable halides can include, for example, F, Cl, Br, or I, or amixture of two or more halides. Lanthanides (or “Ln”) can includelanthanides such as Y, La, Ce, Gd, Lu, Sc, etc. In some embodiments, thecomposition includes a mixture of lanthanide elements. In someembodiments, a scintillator includes a Cs₂LiYCl₆ composition.

In some embodiments, the lithium content of the composition is enrichedto include a Li-6 content above that which is found in naturallyoccurring lithium sources. It should be understood, however, that notall compositions of the invention are enriched. Enrichment refers to achange through processing of a nuclear species mixture found on Earth oras naturally occurring so that the resultant material has a differentmix of nuclear species. In naturally occurring sources of lithium, 93%of the lithium is in the form of Li-7 or ⁷Li, having an atomic weight ofapproximately 7 and includes a nucleus with three protons (defining thechemical species) and four neutrons. Approximately 7% of naturallyoccurring lithium is Li-6 or ⁶Li, which has an atomic weight ofapproximately six, including three protons and three neutrons. Althoughthe chemical properties are substantially similar, the physical (weight)and nuclear properties are significantly different. Of interest in thecompositions of the present invention is that the neutron interactioncross-section of Li-6 is larger than that of Li-7.

Thus, Li-6 enriched compositions of the present invention will includecompositions where the Li-6 content is higher or above that which isfound in naturally occurring lithium sources. Compositions can includelithium with a Li-6 content that is at least about 10% or higher, andwill typically include lithium with a Li-6 content of about 50% or more,and in some instances about 80%, 90%, 95% or more (as well as anyintegral number in the specified ranges).

Most neutron detection applications demand high detection efficiency. Inneutron radiography is important to keep exposure to neutrons low, e.g.,since the interaction of neutrons with many materials results inchemical transformations that alter the properties of these materials bytransmuting elements, and some can become radioactive for long times,this latter factor complicating handling and disposal. In certainapplications, radiation sources of interest may have low-level neutronemissions or low ratio of neutron to gamma-ray emission, therebyrequiring a high-efficiency detector for meaningful analysis oreffective detection. In the nuclear security or monitoring applications,e.g., radiation sources or materials of interest have a low ratio ofneutron emissions relative to gamma-rays, such as one neutron perthousand or more gamma-rays, thus making neutron detection efficiencyand neutron discrimination from gamma rays key factors for detectionand/or analysis applications.

While the detection efficiency of unenriched material might seemingly beaugmented simply by making thicker detectors, this is not necessarilythe case. In practice, detector efficiency is not found to be directlyproportional to scintillator thickness, and merely increasing thicknessproduces diminishing return in terms of efficiency. As an exampleillustrating limitations in merely increasing thickness as a measure toincrease efficiency, the following example is provided. For gamma rays,on first order, for a given scintillator, if a thickness 1× of materialstops 30% of the incoming gamma-rays, then 2× stops 51% and 3×66% and soon. The reason is that substantially all gamma rays interacting in thematerial produce detectable light. Limitations on size/volume ofscintillator that can practically be incorporated into a devicerestricts the usefulness of certain compositions that are unable toprovide a desired detection efficiency with a thickness that ispractical or useful.

In the case of non-enriched Cs₂LiYCl₆, for instance, the Cs and Cl(mainly the latter) do stop some of the neutrons but do notcorrespondingly produce useful neutron detection signal, so as thedetector is made thicker it becomes opaque to neutrons. Thus, asscintillator thickness increases the Li component does decreasingproportion of useful work because the Li-6 component competes forneutrons with the Cl, which as noted above does not interact withneutrons so as to produce a useful signal. As a consequence, because theneutron absorption per unit thickness/length of scintillator materialfor non-enriched Cs₂LiYCl₆ is limited, the maximum neutron detectionefficiency of such material is limited even as the thickness increases.For example, a scintillator composition of non-enriched Cs₂LiYCl₆ maygain some increased neutron detection efficiency as thickness isincreased up to about 10 cm, but further doubling the thickness from 10cm to 20 cm produces minimal further gains in neutron detectionefficiency (see FIG. 1). The maximum detection efficiency for anythickness is approximately 27%. It is noted that, separate for the issueof detection efficiency, as scintillator thickness increases to a rangeof about 10 cm to about 20 cm, practically application of such thickscintillators, particularly in more portable or hand held detectorconfigurations, becomes more limited and even precludes someapplications.

The scintillator compositions of the present invention may include alithium component that is enriched with Li-6, e.g., compared to Li-7.The enriched compositions advantageously allow high-efficiency neutrondetection with relatively thin scintillator configurations, therebyallowing practical application of the compositions in a variety ofdetection devices that would not otherwise be available with acorresponding non-enriched composition. Since in one embodiment, lithiumenriched to the level of 95% Li-6 is available, high-efficiencyscintillators can be produced using enriched material, and canoptionally include a thin scintillator profile. As will be recognized,neutron detection efficiency increases when the ratio of neutronsincident to the scintillator that generate a scintillation event ordetectable signal compared to neutrons incident to the scintillator notgenerating a scintillation event or signal is higher. A“high-efficiency” detector, as referred to herein, can include adetector where the scintillator composition is capable of neutrondetection at an efficiency of about 30% or more, and/or an incorporatingdevice/system will be configured for detection at an efficiency about30% or more. In some embodiments, the scintillator composition iscapable of neutron detection at an efficiency of about 50% or more; insome embodiments, the scintillator composition is capable of neutrondetection at an efficiency of about 75% or more. Often, a detectordevice or system of the present invention will include a neutrondetection efficiency in a range of about 20% to about 80% or greater(and any integral number therebetween). For 95% enriched material themaximum detection efficiency for any thickness is approximately 82%,approximately 3 times higher than for unenriched material, and this isattained for a thickness that is approximately 1/10th of the thicknessof unenriched material.

The terms “thick” and “thin” are used herein in reference to ascintillator thickness or distance from one surface to an opposingsurface (see, also, FIG. 8B, below). In some instances, reference tothickness or thinness refer to scintillators having a thickness of about20 cm or less (e.g., 0.01 cm to about 20 cm, or any integral numbertherebetween), typically less than about 10 cm. In some instances,scintillators are less than about 1 cm and may have portion of about 1mm to about 5 mm in thickness. In some cases, the scintillators have athickness of greater than 1 cm; in some cases, greater than 5 cm; insome cases, greater than 10 cm; and, in some cases, greater than 20 cm.In some cases, the terms thick or thin are used in a relative manner,such as referring to the thickness/thinness of an enriched scintillatorcomposition compared to a corresponding non-enriched composition.

Enriched compositions of the present invention in some embodiments mayprovide significantly greater neutron detection efficiency per unitthickness compared to a corresponding non-enriched composition. In manyinstances, enriched scintillator compositions will provide thinscintillators that are high-efficiency neutron detecting scintillators,and the present invention includes high-efficiency neutron detectionsystems/structures making use of relatively thin enriched scintillatorcompositions.

Increased neutron detection efficiency per unit thickness in enrichedscintillator compositions of the present invention is described withreference to FIG. 1. As can be seen in FIG. 1, in one example, a Li-6enriched Cs₂LiYCl₆ of 1 cm in thickness is capable of detecting throughscintillation about 80% of the neutrons reaching the scintillator, witha maximum detection efficiency of about 82% is reached at about 1.5 cmof material thickness. In contrast, a 10 cm thick correspondingunenriched Cs₂LiYCl₆ material detects approximately 23% of the neutrons,and 20 cm of unenriched material detect 27%. Remarkably, the aboutmaximum detection efficiency (about 27%) of the unenriched Cs₂LiYCl₆that is reached at about 20 cm-thickness, is matched by the Li-6enriched Cs₂LiYCl₆ of the present invention at a thickness of just over1 mm.

One of the valuable characteristics of the scintillator of the presentinvention is the ability to differentiate neutrons from gamma rays. Theprinciple behind discrimination is described with reference to FIGS.2-4. FIG. 2 shows the time course of light emission by gamma rays andneutrons obtained from a small scintillator crystal of Cs₂LiYCl₆ dopedwith Ce. As can be seen, timing profile of a gamma-ray scintillationevent differs compared to neutron scintillation event. For incidentgamma-rays, scintillation is very fast, including a fast light decaywhere 1/e was reached in less than 100 nsec. Neutron scintillation eventexhibits a relatively slower timing profile, the 1/e point being reachedat about 500 ns. The difference in the timing profile between gamma-rayscintillation events and neutron scintillation events can facilitatedifferentiation between gamma-ray detection and neutron detection. Inparticular, such differences enable gamma-ray detection and neutrondetection to be differentiated using pulse shape discrimination (PSD)analysis. PSD analysis, in general, involves comparing the luminescencesignal pulse shape resulting from gamma-ray detection to theluminescence signal pulse shape resulting from neutron detection. Insome embodiments, it may be advantageous to use PSD analysis overrelatively long time periods to differentiate gamma-ray detection andneutron detection. For example, in some embodiments, methods ofdifferentiating gamma-rays from neutrons involve analyzing theluminescence signal over a time of greater than 50 ns; in some cases,over a time of greater than 100 ns; or, in some cases, over a time ofgreater than 150 ns. Relatively long PSD times are particularly usefulin embodiments when the scintillator is relatively thick, for example,greater than 1 cm, greater than 5 cm, etc.

FIG. 3 shows a method to use rise time to effect gamma ray/neutrondiscrimination in larger crystals by placing two time windows from whichto accumulate (integrate) or process the luminescence signal. In theillustrative embodiment, window 1 is on the rise and window 2 on thedecay sides of the time course. In some embodiments, window 1 has a timeduration of at least 50 ns, or at least 100 ns. In some cases, the timeduration for window 1 is less than 150 ns, less than 125 ns, or lessthan 100 ns. Window 1 may be between 0 and 100 ns, as measured from thestart of the luminescence signal. In some embodiments, window 2 has atime duration of at least 50 ns, at least 100 ns (e.g., between 100-125ns), at least 200 ns, or at least 300 ns. In some cases, the timeduration for window 2 is less than 400 ns, less than 300 ns or less than200 ns. Window 2 may be between 100 ns and 500 ns, as measured from thestart of the luminescence signal. Analysis can include a comparison ofwindows 1 and 2 so as to identify a scintillation event as a gamma eventor neutron event. The analysis may include assessing the ratio of thevalue of the integrated signal within the first time window and thesecond time window. The ratio will be different for events due togamma-ray and neutrons and, thus, can be used to differentiate. FIG. 4shows comparison according to one embodiment, where a plot of window2/window 1 vs. window one shows the neutron and gamma-ray events wellidentified.

FIG. 5 shows light emission from of Cs₂LiYCl₆ doped with Ce under gammaray irradiation. There is core valence luminescence (CVL) in the 250-350nm range and light from the dopant in the 350-450 nm range from thegamma rays. Neutrons produce light in the higher range only. A largedetector results in absorption by the dopant of the CVL produced bygamma rays, and re-emission in the 350-450 run range (FIG. 5, maingraph). As described herein, it has been discovered that the lightproduced by gamma rays, in the 250-350 nm range, is strongly absorbed bya large (e.g., thick) scintillator (light output decreases with crystalvolume)(FIG. 5, inset), and re-emitted at the 350-450 nm range (lightoutput increases with crystal volume). This being the case, a thickdetector cannot take advantage of conventional methods for neutron/gammaray discrimination based on decay time measurements, which aregamma/neutron discrimination. Because the light is being absorbed by theCe dopant, we have discovered that to make large volume detectors Celevels will preferably be low. The non-limiting examples shown herecontain 0.05% of Ce as a dopant.

Because of its relatively low light yield for gamma rays (22,000photons/MeV for Cs2LiYCl6:Ce) compared to scintillators such as LaBr3which yield 60,000 to 90,000 photons/MeV, it would be expected that theenergy resolution of Cs₂LiYCl₆ for gamma rays will be poor, making thematerial less attractive for simultaneous gamma and neutron detection.Indeed, energy resolution of 7-11% for non-enriched Cs₂LiYCl₆ for the662 KeV peak of Cs-137 has been reported previously (Bessiere et al,Nuclear Instruments and Methods in Physics Research A 537 (2005) pp.242-246). Thus, while Cs₂LiYCl₆ has been suggested for use indifferential detection of gamma-rays and neutrons, factors such asmodest to poor energy resolution has limited applicability in thiscontext. According to the present invention, however, it has beendiscovered that the proportionality of response of Cs₂LiYCl₆:Ce is veryhigh (+/−5%) (e.g., FIG. 6) and as a consequence its energy resolutionis surprisingly good, 4% for 662 keV (FIG. 7). As such, the Li-6enriched Cs₂LiYCl₆ compositions of the present invention demonstrateexcellent energy resolution suitable for simultaneous or differentialdetection of gamma-rays and neutrons.

As indicated above, scintillator compositions disclosed herein caninclude a dopant or a mixture of dopants. Dopants can affect certainproperties, such as physical properties (e.g., brittleness, etc.) aswell as scintillation properties (e.g., luminescence, etc.) of thescintillator composition. The dopant can include, for example, cerium(Ce), praseodymium (Pr), lutetium (Lu), lanthanum (La), europium (Eu),samarium (Sm), strontium (Sr), thallium (Tl), chlorine (Cl), fluorine(F), iodine (I), and mixtures of any of the dopants. Where certainhalides are included as dopants, such dopants will be present in thescintillator composition in addition to those halide(s) alreadyotherwise present in the scintillator compound. The amount of dopantpresent will depend on various factors, such as the application forwhich the scintillator composition is being used; the desiredscintillation properties (e.g., emission properties, timing resolution,etc.); and the type of detection device into which the scintillator isbeing incorporated. For example, the dopant is typically employed at alevel in the range of about 0.01% to about 20%, by molar weight. Incertain embodiments, the amount of dopant is in the range of about 0.01%to less than about 100% (and any integral number therebetween), or lessthan about 0.1%, 1.0%, 5.0%, or 20% by molar weight.

The scintillator compositions of the invention may be prepared inseveral different forms. In some embodiments, the composition is in acrystalline form (e.g., monocrystalline). Scintillation crystals, suchas monocrystalline scintillators, have a greater tendency fortransparency than other forms. Scintillators in crystalline form (e.g.,scintillation crystals) are often useful for high-energy radiationdetectors, e.g., those used for gamma-ray or X-ray detection. However,the composition can include other forms as well, and the selected formmay depend, in part, on the intended end use of the scintillator. Forexample, a scintillator can be in a powder form. It can also be preparedin the form of a ceramic or polycrystalline ceramic. Other forms ofscintillation compositions will be recognized and can include, forexample, glasses, deposits, vapor deposited films, and the like. Itshould also be understood that a scintillator composition might containsmall amounts of impurities. Also, minor amounts of other materials maybe purposefully included in the scintillator compositions to affect theproperties of the scintillator compositions.

Scintillator compositions can be substantially pure (e.g., about 99%scintillator composition or greater) or may contain certain amounts ofother compounds or impurities. In some cases, impurities may originate,for example, with starting materials for composition preparation.Typically, impurities constitute less than about 0.1% by weight of thescintillator composition, and often less than about 0.01% by weight ofthe composition. In some instances, minor amounts of other materials maybe purposefully included in the scintillator compositions. For example,minor amounts of other rare earth metals, oxides can be added to affectscintillation properties, such as reduce afterglow, and the like.Scintillator compositions can include single halide compositions as wellas mixed halide compositions, e.g., where the term halide includes amixture of two or more halides.

Methods for making crystal materials can include those methods describedherein and may further include other techniques. Typically, theappropriate reactants are melted at a temperature sufficient to form acongruent, molten composition, with operative melting temperature(s) atleast partially depending on the identity of the reactants themselves(see, e.g., melting points of reactants). Non-limiting examples of thecrystal-growing methods can include certain techniques of theBridgman-Stockbarger methods; the Czochralski methods, the zone-meltingmethods (or “floating zone” method), the vertical gradient freeze (VGF)methods, and the temperature gradient methods. See, e.g., (see also,e.g., “Luminescent Materials”, by G. Blasse et al, Springer-Verlag(1994) and “Crystal Growth Processes”, by J. C. Brice, Blackie & Son Ltd(1986)).

In the practice of the present invention, attention is paid to thephysical properties of the scintillator material. In particularapplications, properties such as hygroscopy (tendency to absorb water),brittleness (tendency to crack), and crumbliness should be minimal.

TABLE I Properties of Scintillators Wavelength Light Output Density OfEmission Rise-time Material (Photons/MeV) (g/cm³) (nm) (ns) NaI(T1)38,000 3.67 415 >10 CsI(T1) 52,000 4.51 540 >10 LSO 24,000 7.4 420 <1BGO 8,200 7.13 505 >1 BaF₂ 10,000 4.88 310, slow <0.1 ~2,000 220, fast GS0 7,600 6.7 430 ~8 CdW0₄ 15,000 8.0 480 YAP 20,000 5.55 370 <1

Table I provides a listing of certain properties of a number ofscintillators. As shown, Li-6 enriched Cs₂LiLn:Z₆ compositions of thepresent invention demonstrate a useful light emission spectrumcomparable to other commercially available scintillators. Table IIfurther provides certain properties for a Cs₂LiY:Cl₆. doped with 0.05%of Ce scintillator composition, according to an embodiment of thepresent invention.

TABLE II RbGd₂Br₇:Ce Li- Li₆Gd Property Cs₂LiYCl₆:Ce (RGB) Lil:EuGlass:Ce (BO₃)₃:Ce λ _(em,) nm 373 420 470 395 400 Light yield, 1neutron 73,000 <5,000 50,000 ~6,000 50,000 photons per 1 MeV γ-ray22,000 56,000 12,000 ~4,000 14,000 decay time constants, ns 1*, 25, 200045, 400 1,400 75 200, 700 density ρ, g/cm³ 3.31 4.8 4.1 2.5 3.5 Pulseshape γ-¹n yes no no no no discrimination

As set forth above, scintillator compositions of the present inventionmay find use in a wide variety of radiation detection and processingapplications and structures. Thus, the present invention includesmethods and structures for detecting energy radiation (e.g., gamma-rays,X-rays, neutron emissions, and the like) with a scintillation detectorincluding the scintillation composition of the invention.

FIG. 8A is a diagram of a radiation detection system or apparatus of thepresent invention. The detector system 10 includes a scintillator 12optically coupled to detector assembly including a light photodetectorassembly 14 or imaging device. The detector assembly of system 10 caninclude a data analysis or computer system 16 (e.g., data acquisitionand/or processing device) to process information from the scintillator12 and light photodetector 14. In use, the detector 10 detects energeticradiation emitted form a source 18.

A system as in FIG. 8A containing the scintillator composition(scintillator 12) of the present invention is optically coupled to thedetector assembly (e.g., photodetector 14) and can include an opticalwindow that can be disposed, e.g., at one end of the enclosure-casing.The window permits radiation-induced scintillation light to pass out ofthe scintillator composition assembly for measurement by the photondetection assembly or light-sensing device (e.g., photomultiplier tube,etc.), which is coupled to the scintillator assembly. The light-sensingdevice converts the light photons emitted from the scintillator intoelectrical pulses or signal that are output and may be shaped,digitized, or processed, for example, by the associated electronics.

A data analysis, or computer system thereof can include, for example, amodule or system to process information (e.g., radiation detection dataor signals) from the detector/photodetectors can also be included in aninvention assembly and can include, for example, a wide variety ofproprietary or commercially available computers, electronics, or systemshaving one or more processing structures, a personal computer,mainframe, or the like, with such systems often comprising dataprocessing hardware and/or software configured to implement any one (orcombination of) the method steps described herein. Any software willtypically comprise machine readable code of programming instructionsembodied in a tangible media such as a memory, a digital or opticalrecording media, optical, electrical, or wireless telemetry signals, orthe like, and one or more of these structures may also be used totransmit data and information between components of the system in any ofa wide variety of distributed or centralized signal processingarchitectures.

The detector assembly typically includes material formed from thescintillator composition described herein (e.g., one or morescintillator crystals). The detector further can include, for example, alight detection assembly including one or more photodetectors.Non-limiting examples of photodetectors include photomultiplier tubes(PMT), photodiodes, CCD sensors, image intensifiers, and the like.Choice of a particular photodetector will depend in part on the type ofradiation detector being fabricated and on its intended use of thedevice. In certain embodiments, the photodetector may beposition-sensitive.

FIG. 8B shows a scintillator as in scintillator 12 illustrated in FIG.8A. Scintillator 12 includes a Cs₂LiLn:Z₆ composition as describedabove. Various sizing, shapes, dimensions, configurations ofscintillator 12 may be selected depending on intended use and/or systemin which the scintillator 12 is incorporated. Scintillator 12 includes atop side 18 and an opposing side (not shown) with a thickness (“T”)measuring between the top side 18 or surface of the scintillator 12 andthe opposing side or surface. Scintillator 12 is shown coupled to asubstrate 20, which may be selected from a variety of substrates.Non-limiting substrate composition examples may include amorphouscarbon, glassy carbon, graphite, aluminum, sapphire, beryllium, or boronnitrate. A substrate may include a fiber optic plate, prism, lens,scintillator, or photodetector. The substrate can be a detector deviceor portion or surface thereof (e.g., optical assembly, photodetector,etc.). The substrate can be separate from a detector device and/orcomprise a detector portion (e.g., scintillator panel) that can beadapted to or incorporated into a detection device or assembly. In oneembodiment, the scintillator is optically, but not physically, coupledto a photodetector.

The detector assemblies themselves, which can include the scintillatorand the photodetector assembly, can be connected to a variety of toolsand devices, as mentioned previously. Non-limiting examples includenuclear weapons monitoring and detection devices, well-logging tools,and imaging devices, such as nuclear medicine devices (e.g., PET).Scintillator compositions of the present invention, e.g., due tohigh-detection efficiency and/or relatively thin profile or sizingdescribed above, can be incorporated into smaller or more compactdevices or systems, including hand-held probes, detectors, ordosimeters, portal monitoring structures, and the like. Varioustechnologies for operably coupling or integrating a radiation detectorassembly containing a scintillator to a detection device can be utilizedin the present invention, including various known techniques.

The detectors may also be connected to a visualization interface,imaging equipment, or digital imaging equipment. In some embodiments,the scintillator may serve as a component of a screen scintillator. Forexample, powdered scintillator material could be formed into arelatively flat plate, which is attached to a film, such as photographicfilm. Energetic radiation, e.g., X-rays, gamma-rays, neutron,originating from a source, would interact with the scintillator and beconverted into light photons, which are visualized in the developedfilm. The film can be replaced by amorphous silicon position-sensitivephotodetectors or other position-sensitive detectors, such as avalanchediodes and the like.

Neutron radiographic devices represent another important application forinvention scintillator compositions and radiation detectors.Furthermore, geological exploration devices, such as well-loggingdevices, represent an important application for these radiationdetectors. In such an embodiment, gamma-rays can be detected, which inturn provides an analysis of geological formations, such as rock stratasurrounding the drilling bore holes.

Specific aspects or dimensions of any of the compositions, devices,systems, and components thereof, of the present invention may readily bevaried depending upon the intended application, as will be apparent tothose of skill in the art in view of the disclosure herein. Moreover, itis understood that the examples and embodiments described herein are forillustrative purposes only and that various modifications or changes inlight thereof may be suggested to persons skilled in the art and areincluded within the spirit and purview of this application and scope ofthe appended claims. Numerous different combinations of embodimentsdescribed herein are possible, and such combinations are considered partof the present invention. In addition, all features discussed inconnection with any one embodiment herein can be readily adapted for usein other embodiments herein. The use of different terms or referencenumerals for similar features in different embodiments does notnecessarily imply differences other than those which may be expresslyset forth. Accordingly, the present invention is intended to bedescribed solely by reference to the appended claims, and not limited tothe preferred embodiments disclosed herein.

What is claimed is:
 1. A scintillator comprising a Cs₂LiLn Halidecomposition, wherein Ln is selected from one or more of Y, La, Ce, Gd,Lu and Sc, wherein the scintillator is capable of neutron detection atan efficiency of greater than 30%, wherein the scintillator is enrichedwith Li⁻⁶.